I think there was some sense to not requiring life jackets on seaplanes. They’re much more confined spaces than most pleasure boats, not to mention that you’re usually on a boat rather than in it. The flooding is also usually just about instant as the airplane rolls over.
Seems common for reactive legislation to not actually fix the situation that’s being reacted to. Requiring shoulder harnesses during takeoff and landing (which is the case in the US) would have actually kept the deceased passenger conscious to escape, as said in the report. But they didn’t change that law.